This invention relates to the polyphase electric machines, being motors and generators. The invention is described with reference to motors, particularly three phase motors, but as indicated the invention is not so restricted.
A limitation of presently known induction motors is that, since speed of rotation is governed by the frequency of the A.C. supply and this frequency is fixed, it is not easy to vary the motor speed by such frequency variation. Complex and expensive circuits do exist for varying the frequency of the A.C. supply being fed to the stator of the induction motor, these circuits being located between the source and the motor winding. Conventionally, such motors require that the stator supply be a relatively pure sinusoidal wave in order to achieve efficient operation. Circuitry to achieve this, especially for high powered machinery is also complex and expensive. A more efficient usage of electrical power is achieved by having a motor which is speed variable according to load requirements in comparison with a motor which is operative at a relatively fixed speed or speeds or is otherwise stationary, depending on load requirements.
Another disadvantage with induction motors is that when heavily loaded, they draw excessive currents as the motor slows down, which currents can result in motor burn out unless the motor is protected by auxiliary equipment. Such motors must have a high breakaway torque to running torque ratio to prevent motor damage in the event of motor overload, as a result the flux density must be maintained at non-optimum levels during normal operation. This relatively low flux density during normal operation is also necessitated by potential input voltage variations. Because the flux density must be kept relatively low, the motor size must be substantially larger than would theoretically be necessary in an ideal motor in order to obtain the desired output horsepower.
Another problem encountered in conventional induction motors is the high starting current inherent to their operation. This also causes the flux density to be higher at low loads than is actually necessary for efficient operation at such loads.
Yet, another problem which is known to exist is that of designing a motor to run through its normal load range efficiently, to provide high power factor, and simultaneously to provide high starting torque and high breakdown torque when required for particular applications.